Lab Report 2

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1 Spectrophotometric Determination of The Reaction Order and Rate Constant for a Chemical Reaction, Experiment Number 07 Sydney Nierstedt and Manar Maybrouk, 11/1/22 Department of Chemistry, Binghamton University, Binghamton, NY 13902

2 Introduction The kinetics of a decomposition reaction are observed in the laboratory exercise, Spectrophotometric Determination of The Reaction Order and Rate Constant for a Chemical Reaction, using previous knowledge of a spectrophotometer. Sodium hypochlorite, or NaOCl, is an oxidizing agent in household bleach. It is a very reactive and unstable chemical compound. NaOCl is produced through a method called the Hooker Process. In this process Cl 2 gas is passed through cold dilute NaOH solution, resulting in NaOCl(2). Cl 2 +2NaOH->NaCl+NaOCl+H 2 O When dissolved in water Sodium hypochlorite forms Hypochlorous acid, or HOCl, responsible for the bleaching effect in household bleach. Bleaching is a process that causes a colored compound to become colorless using a chemical reaction. Hypochlorous acid can break the bonds of dye, preventing the molecule from absorbing visible light, therefore bleaching it(3). FD&C Blue Dye No. 1, or Erioglaucine disodium salt, is a blue dye found commonly in candy coating and other blue-color solutions. NaOCL will oxidize this colored dye to bleach it(1) Measuring the change in absorbance, A, will allow the decomposition reaction to be followed. The Vernier SpectroVis Plus Spectrophotometer will be used to measure the absorbance and Beer’s Law will be used to calculate the concentration of the FD&C Blue Dye solution. M dilute V dilute =M Concentration V concentration is used to calculate the amount of solute in each dilution. M stands for molarity and V stands for volume. A=cl[dye]

3 L is the thickness of the solution and c is a unique constant. Beer’s Law is used to show that A is proportional to [dye]. Square brackets represent concertation. The rate of reaction will be calculated using: Rate=k[dye] m [OCL - ] n The rate of reaction will be represented by the disappearance of the colored species in the solution. M and n are the order of reaction depending on each reagent and k is the rate constant for the reaction at room temperature. Summing m and n will result in the overall rate of reaction. Since there is excess bleach concentration very little OCL - will react with the dye and the concentration of OCl - can be considered constant, changing the rate law to: Rate=k’[dye] m K’=k[OCl - ] n Using these equations graphically k’ can be determined as well as m. For zero order reactions integrating the rate equation will give [dye]—k’t+[dye] 0 . This means that [dye] plotted against t will produce a straight line, resulting in a zero order reaction. First order reactions have an integrated rate law of ln[dye]=-k’t+ln[dye] 0. This order will produce a straight line when ln[dye] is plotted against t The rate equation for a second order reaction is -d[dye]/dt=k’[dye] 2 and the integrated equation is 1/[dye]=k’t+1/[dye] 0. 1/[dye] plotted against t will produce a straight line for second order reactions. These three graphical methods determine the reaction order for the dye and the k’, or m. The slope of each straight line is equal to the rate constant, and the intercept is equal to the reciprocal of the dye’s original concentration. The rate constant determined from the graph must

4 be reported in terms of concentration. [dye]=c x A is used to determine the concentration, c equaling the slope. The reaction order for OCl - and k is determined using k’=k[OCl] n When [OCl] is doubled, if k’ does not change the reaction is zero order, if k’ doubles the reaction is first order, and if k’ quadruples the reaction is second order. Using the value of n, the value of k can be determined for each experiment. These k values allow the rate constant of the reaction to be calculated by calculating the average of k. The overall rate constant is calculating by adding m and n together. The objectives of this experiment are to study the kinetics of a chemical reaction using a spectrophotometer, to determine the overall order of a chemical reaction, and to determine the rate constant of a chemical reaction(1). It can be hypothesized that the overall rate can be calculated by adding the rate of the dye concentration and the rate of the OCl - concentration together.

5 Experimental Section This experiment is broken into two parts. Part one of the lab involved the preparation of Blue#1 dye solutions using dilutions and Beer’s Law. First the molarity of the blue dye was calculated using the molar mass of the blue dye. Blue#1 dye=20.0mg/1000mg=0.020g 0.020g Blue#1 dye(1mol/792.84g)=2.52e-5mol/L Two burets were used, one containing Blue#1 stock solution and the other containing distilled H 2 O. Four dilutions were created. The first consisted of 1.00mL Blue#1 stock solution and 19.00 mL distilled water. The second contained 2.00mL Blue#1 stock solution and 18.00mL distilled water. The third one had 4.00Ml Blue#1 stock solution and 16.00Ml distilled water. The fourth contained 8.00mL Blue#1 stock solution with 12.00mL distilled water. Dilution 1 was observed first, and an operating wavelength was chosen from a peak in the Loggerpro graph( see figure 1) . The absorbance of the other three dilutions were observed and recorded using the Vernier SpectroVis Plus Spectrophotometer and LoggerPro at the operating wavelength( see Table 1). The kinetics of the Blue#1 dye and the OCl - reaction were studied in part two. Three dilutions were created using a 1.0% bleach stock solution. The first dilution was 0.25% bleach solution, the second was 0.50% bleach solution, and the third was 1.00% bleach solution. The volumes of each solution were 1.25mL bleach stock solution and 3.75mL distilled water for the first, 2.50mL bleach stock solution and 2.50mL distilled water for the second, and 5.00mL bleach stock solution with 0.00 mL of distilled water for the third( see Table 2) . The molar concentrations for each dilution were calculated using M dilute V dilute =M Concentration V concentration. (1%)(1.25ml)=M(5ml)

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